Totally anecdotal, but here it is for your consideration. This was a while ago, before the while iPhone encryption fiasco.
At a wedding ~5 or 6 years ago, I was seated next to a guy that was a full-time technician for the NY state police. He said his job included a lot of tasks, but one of those tasks was getting evidence from seized electronics, such as cameras, GPSs, thumb drives, memory cards, phones, computers, etc. for the detectives to examine, and later hand over to the prosecutors if the case went forward.
I asked him whether he has encountered TrueCrypt lockers and computers locked with Apple FileVault2, and if so, how his office handles such things. I think I remember FileVault2 has just been released when we had this conversation, which is why I had thought to ask him.
He did not get into specifics, but his briefly explained: They have some brute-force dictionary-based tools for unlocking the easy passwords. They generally don't bother spending any significant amount of time on unlocking this stuff, their goal is to get as much evidence as fast as reasonably possible, not to get all the evidence possible. So it sounded like most of the time, if he can't unlock it within a few minutes or half hour, he moves on to the next thing. If the crime is serious, or the suspect is a notorious wanted criminal, they will hand the stuff over to the FBI. He said in his time, that had only occurred once with him, and it was an "America's Most Wanted" level of criminal.